With the increased number of patients surviving prolonged immunosuppression, there has been an increase in invasive aspergillosis. Corticosteroids and neutropenia appear to be the major risk factors for invasive disease, presumably because of interference with the ability of host leukocytes to both inhibit germination of conidia and kill tissue invasive hyphae. When these defense mechanisms fail, inhaled conidia disseminate outside the lungs and germinate to form hyphae which invade blood vessel walls, inducing thrombosis, hemorrhage, necrosis and infarction of tissues. Thus, interactions of Aspergillus conidia and hyphae with leukocytes and vascular endothelium are critical factors in the pathogenesis of aspergillosis. Preliminary studies established that Aspergillus hyphae were killed by neutrophils and monocytes and degined the microbicidal mechanisms responsible. In contrast to leukocytes, though endothelial cells actively endocytosed Aspergillus conidia or hyphae, the fungi were not killed in the process, but continued to grow. New assays were developed to quantitate germination and killing of Aspergillus conidia, permitting microbicidal systems and serum inhibitory activity for Aspergillus to be defined. In addition, Aspergillus hyphae were noted to release factors that were chemotactic for neutrophils, in addition to stimulating generation of chemotactic factors from serum and alveolar macrophages. In an experimental mouse model, early host defenses preventing disseminated aspergillosis after upper respiratory challenge were dependent largely upon early killing of conidia by alveolar macrophages, a process impaired by coerisone-treatment. In addition, the failure of neutrophils to kill ingested Aspergillus conidia was found to be due to failure of conidia to stimulate an optimum respiratory burst, plus an increased resistance of conidia to microbicidal oxidants produced by neutrophils. To characterize the basis of Aspergillus interactions with host cells, training is planned to permit utilization of: 1) molecular biological techniques to define surface antigens of Aspergillus that determine interactions with leukocytes or endothelial cells; 2) biochemical characterization of components of Aspergillus and host cells involved in these interactions; 3) fluorescent probes in elucidating the effects of Aspergillus conidia and spores on neutrophil activation; and 4) analysis of interactions of Aspergillus spores and hyphae with endothelial cells and leukocytes together.